One problem which is especially addressed in the present application is the control of a characteristic of output laser light especially of at least one of intensity, signal-to-noise ratio, wall-plug efficiency, departing from a laser system as addressed above. Nevertheless the solution of this object may be applied more generically on laser systems where especially construction, compactness power consumption and accurate evaluation are prevailing considerations.
Thus the present invention is directed on a method for producing laser light with a desired characteristic of the output laser light. This is accomplished according to the present invention in that there is generated laser light in a spectrum range. The laser light is filtered with at least one filter characteristic and the spectral location of the at least one filter characteristic is shifted to establish the desired characteristic.
Instead of providing stabilizing measures within a laser system so as to properly control e.g. to keep constant, parameters which do affect the addressed characteristic of output laser light which measures customarily necessitate significant constructional efforts and do consume additional power as e.g. for cooling, negative feedback controlling purposes, the desired characteristic is achieved and maintained by controllably shifting the spectral location of a filter characteristic downstream the laser source which allows adjustment of the addressed characteristic.
In one embodiment of the method according to the present invention the spectral location of the spectrum range of the laser light as generated shifts in dependency of temperature and the addressed method further comprises the step of shifting the spectral location of the at least one filter characteristic matched with the shift of the spectral location of the spectrum range of laser light.
Thereby, it becomes possible whenever the spectral range of the generated laser light, which comprises the predominant laser light wavelength, shifts due to of temperature and would, by such spectral shift, be subjected to varying transmission at the filter characteristic kept stationar, to cope with the resulting variation of the characteristic considered. This by having the spectral location of the filter characteristic shifted in a matched manner with the addressed shift of the laser light spectral band.
In other words the filter characteristic is made to follow the addressed spectral band as it varies with respect to spectral position.
In a further embodiment the shift of the spectral location of the spectrum range of laser light in dependency of temperature is controlled by shifting a further spectral location of a stabilizing filter characteristic in dependency of temperature. Here the addressed matching is performed between the shift of the spectral location of the one filter characteristic as addressed above and the further spectral location of the further filter characteristic.
Taking e.g. a lasering device which emits light within a spectral band. The addressed stabilizing filter characteristic which is (see Definition of stabilizing filter) a narrow pass-band filter characteristic, determines out of the spectral band a narrower spectral band of the generated laser light at which emission is stabilized. When such filter characteristic is shifted spectrally as a function of temperature within the spectral band of light emitted from the unstabilized lasering device, the even narrower band-width of the generated laser light is spectrally shifted, too. Thereby the one filter characteristic of the filter addressed above is spectrally shifted, matched with the spectral shift of the stabilizing filter characteristic. Thus temperature caused variations of the spectral location of the generated laser light is caused by the spectral shift of the stabilizing filter characteristic and as the one filter characteristic is spectrally dislocated matched with the stabilizing filter characteristic, temperature influences of the desired characteristic are substantially avoided.
In a further embodiment at least one temperature is sensed.
The at least one temperature as sensed is converted into a mechanical signal. Shifting of the spectral location of the at least one filter characteristic is performed in dependency of the mechanical signal. Thereby it is taken into account that a predominant part of optical filters applied to laser systems have filter characteristics which are defined by geometric entities as by thickness of interference layers, period of gratings etc. Therefore, the addressed spectral shift of the filter characteristic is performed by acting upon at least one such geometric entity which is performed mechanically, thereby requiring a temperature-to-mechanical conversion for making the addressed spectral shift dependent from temperature.
In a further embodiment the at least one filter characteristic is provided by at least one geometric entity of at least one filter element and a mechanical signal is applied to said filter element so as to affect the geometric entity. Thereby by such mechanical signal as of a force or a momentum, one or more than one geometric entities as of grating period, thickness of layers, position and shape of material interfaces is affected and varied, entities which govern filter characteristic of the optical filter element.
In a further embodiment the addressed temperature sensing is performed remote from a filter element with the filter characteristic. Thereby a temperature prevailing at a location remote from such filter element may be applied for spectrally shifting the filter characteristic at the addressed filter element. In one embodiment a temperature to mechanical conversion is performed remote too and the result mechanical signal is applied to the filter element to controllably shift the addressed spectral location of its filter characteristic. We call the technique of remote sensing temperature for the addressed spectral location shift “active”.
In a further embodiment the temperature is sensed by the addressed filter element itself and the mechanical signal and/or a variation of an optical parameter as of index of refraction of a material is generated. The mechanical signal is generated by variation of a geometric entity at the filter element caused by temperature change which entity governs the spectral location of the filter characteristic. Thereby it is exploited that solid materials exhibit a variation of their geometric and/or optical parameters in dependency of temperature which is exploited to shift the spectral location of the filter characteristic of a filter element.
In a further embodiment the at least one filter characteristic is realized in or at an optical fibre.
Thereby a significant improvement with respect to compactness of a respective laser system is achieved.
In a further embodiment of the method according to the present invention, the at least one filter characteristic is realized by at least one of dielectric material layers, surface gratings, volume gratings or Bragg gratings. This is especially suited when the addressed filter element is realized as in or at an optical fibre.
In a further embodiment the method according to the present invention comprises amplifying the laser light before it impinges on one of the at least one filter characteristics the spectral location of which being shiftable. Thereby by such amplification normally also noise is amplified i.e. light outside of the spectral range of laser light. By providing downstream such amplifying the addressed at least one filter characteristic, which normally will be a pass-band characteristic, on one hand signal-to-noise ratio is improved and such improvement is maintained even as the spectral range of the generated laser light spectrally shifts.
In a further embodiment the amplifying just addressed is performed by an active fibre amplifier. Thereby it becomes apparent that the noise which was just addressed is at least substantially generated by amplified spontaneous emission ASE. This, on one hand makes the addressed downstream filtering most desirable to improve signal-to-noise ratio of output laser light and—on the other hand—the addressed shifting of spectral location of the respective filter characteristic matched with such shifting of the spectral range of laser light as generated, maintains the targeted characteristic as e.g. desired signal-to-noise ratio, independent from spectral range shift of the generated laser light and irrespective of the origin of such shift.
In a further embodiment the gain of amplifying is modulated. Thereby additionally to the addressed shifting of spectral location of the filter characteristic, a further improvement for achieving and maintaining a desired characteristic as intensity and/or signal-to-noise ratio at the output laser light is realized. In a further embodiment the gain is modulated by at least one of intensity of pumping light, of spectrum of pumping light, of pulse-width-modulation of pulsed pumping light and of shifting spectral position of a filter characteristic and of length of active fibre material.
In a further embodiment of the method according to the present invention, generating laser light comprises generating laser light by a laser diode. Thereby a further improvement with respect to compactness and robustness and possibly also with respect to power consumption is achieved.
In a further embodiment the laser light as generated is generated in a pulsed manner. Thereby the possibility is opened to apply such pulsed laser light with the desired characteristic for target designator purposes or range finding purposes thereby evaluating pulsed laser light reflected from targets and, in one embodiment, evaluating multiple reflected pulses.
In a further embodiment of the method according to the present invention and generating the addressed laser light in a pulsed manner the pulsed laser light is amplified in a pulsed pumped manner whereby pulsating pumping of amplifying is synchronized with generating the laser light in pulsed manner.
Thereby signal-to-noise ratio as one characteristic of the output laser light is significantly increased.
In a further embodiment laser light dependent from the laser light as generated is emitted and laser light dependent on the emitted laser light is received at one common laser input/output port which especially in context with range finding applications of the addressed method further improves constructional compactness of the respective laser system.
In one embodiment laser light dependent from laser light as generated, is guided by an optical fibre to a transmitter optic. Thereby the divergence of the laser beam output from the transmitter optic is determined by appropriately conceiving the end of the fibre adjacent to the transmitter optic. Different approaches to do so are addressed in the detailed description part. By doing so a significant saving of lenses is achieved which leads to further advantages with respect to compactness, robustness and price of a respective laser system.
In a further embodiment the transmitter optic is also a receiver optic for laser light and, still in a further embodiment, the addressed optical fibre is an active optical fibre.
Still in a further embodiment the laser light as generated is guided up to a laser output port substantially exclusively in optical fibres. Thereby on one hand constructional compactness is significantly increased, opening the possibility to perform the addressed method in a portable and even handheld device.
Under a further aspect of the present invention there is proposed a method of laser range finding or laser target designating which comprises generating laser light by the method as was addressed in a pulsed manner and directing laser light dependent on said laser light generated towards a target.
In a further embodiment of the just addressed method of laser range finding, multiple laser light pulses as received and as reflected from a target are evaluated.
In a further embodiment of the methods according to the present invention the characteristic to be brought on a desired value as e.g. intensity and/or signal-to-noise ratio of laser light, is sensed and the spectral location of the at least one filter characteristic is shifted as an adjustment in a negative feedback controlled manner.
In one embodiment the desired characteristic is at least one of laser light intensity, signal-to-noise ratio and wall-plug efficiency.
Further the present invention is directed on a laser system with a laser light source the output thereof being operationally coupled to an input of at least one optical filter. The optical filter has a spectral filter characteristic. The optical filter has further a control for the spectral position of the filter characteristic.
Specific embodiments of this laser system are further defined by the claims.
Attention is drawn on the fact that the content of the European application no. 05 000 669.1 dated Jan. 14, 2005 as well as the content of the European application no. 04 029 867.1 dated Dec. 16, 2004 upon which the present application resides with respect to priority, is considered as a part integrated by reference to the present disclosure.